1. Field of the Invention
The present invention is directed to an optical fiber strain sensor that uses a ring or linear laser cavity to detect strain and, more particularly, to a cavity that uses an active doped fiber section to set up multiple oscillation modes in the cavity which create beat frequencies used to determine fiber strain and which includes a section of the fiber which is insensitive to thermally created apparent strain.
2. Description of the Related Art
There is a significant need for low cost sensors that can be embedded in or surface-mounted on structural materials to monitor static and dynamic strain. Some examples of the need for static strain sensing include: monitoring the deformation of shape-critical parts of spacecraft (antennas, interferometric booms, etc.) caused by thermal excursions, slew maneuvers or debris damage; detecting the onset of cracking in airplane wings; monitoring the strain at joints between structural elements in submersible vehicles induced by static pressure at depth; and detecting deformation and initial damage in a wide range of structures, which might include runways, carrier decks, ship hulls, bridges, and even buildings.
In many of these applications, there is an additional requirement that the electronic instrumentation associated with the sensor be low power and light weight and that the elements of the sensor be low cost and readily available. Also, it would be desirable that the sensor report a measurand related to absolute strain (rather than strain relative to some initial value), so that the sensor need not be constantly interrogated to maintain calibration.
Typical fiber optic strain sensors in use today are based on the principle of interferometry where the sensor consists of a sensing fiber leg and a reference fiber leg. The length of the sensing leg is changed by the strain applied to the fiber. The optical phase shift between the two legs is a measure of the length change and the strain. In general, interferometric sensors cannot provide measurement of the absolute length of the sensing leg, only measurement of the change in length, i.e., strain, relative to a baseline measurement made when the sensor system was activated. No knowledge of strain of length changes can be obtained for periods when the instrumentation is turned off or disconnected from the sensing fiber. Subsequent strain measurements are made relative to a new baseline obtained when the instrumentation is reactivated.
More recently strain sensors using fiber reentrant loops have been proposed. Use of a loop of fiber to detect strain has been reported in J. Lightwave Technology, 8, 1273 (1990) by R. O. Claus and coworkers at the Virginia Tech Fiber and Electro-Optics Research Center. In the embodiment discussed in the Claus paper, a pulse is inserted into the loop via a coupler. Each time the pulse travels around the loop, the coupler extracts and outputs a very small portion of the light (25-30 dB). These couplers, which are made by attaching a 50 micron core injection-tapoff fiber to a 200 micron core fiber in the loop, are a specialty product of Litton Scientific. A detector on the output senses a decaying series of pulses whose frequency is the inverse of the optical path length of the loop. The intensity of the pulse is attenuated by the loss in the fiber loop and the coupler (typically 0.3-0.4 dB) in addition to the amount of light extracted by the tap-off coupler on each pass. In addition, the breadth of the pulse increases due to the dispersive characteristics of the 200 micron core multimode fiber used in the loop. Although use of single mode fiber would significantly reduce the dispersion, it is extremely difficult to fabricate low loss, asymmetric tap-off couplers from single mode fiber; thus, these reentrant loop schemes have generally used a large core multimode fiber and have operated at 0.85 or 1.3 .mu.m. Claus et al. have reported that attenuation and dispersion limit the sensitivity to a fiber elongation of 5-10 .mu.m.
A second embodiment was also proposed and demonstrated by Claus et al. Here a partially reflective splice (once again a hand-fabricated item) is introduced into the loop. A specially-modified optical time domain reflectometer (OTDR) sends a pulse into the loop and monitors the weak reflections from the splice as the pulse transits the loop. The OTDR must be modified to increase its dynamic range. In addition to the problems with attenuation and dispersion described above, this technique requires an OTDR, which is a bulky, expensive piece of laboratory equipment, and the use of special tap-off couplers to maintain as much of the optical signal circulating in the loop as possible.
These prior art strain sensors, in addition to being expensive to make, are sensitive to temperature changes that cause inaccurate strain measurements. This cross sensitivity to temperature is the most significant limitation in optical strain sensors developed to date. Temperature variations can cause changes in the optical path length, through direct changes in the physical length of the sensor and through changes in the refractive index. Changes in optical path length appear to the measurement instruments as strain. These temperature variations limit the sensitivity of the sensor.
The reentrant loop sensors provide absolute measurement but only with limited sensitivity. The interferometric sensors developed to date provide relative strain measurement.
What is needed is an optical fiber strain sensor that is independent of thermal apparent strain (that is, athermal), inexpensive, and capable of intermittent absolute strain measurements.